Nitride compounds of the GaN, AlN, AlGaN, AlGaInN type, etc., have semi-conducting properties of the large direct gap type, notably used for applications in electronics or optoelectronics at a microscopic or nanoscopic scale. In particular, GaN has a direct gap of 3.4 eV that gives it the property of emitting light in the ultraviolet. Combined with materials such as aluminum or indium in order to modulate its gap, it is possible to make light-emitting diode or laser diode type structures emitting in the blue, the violet and the ultraviolet. One of these alloys emitting in the ultraviolet may also be combined with phosphorus in order to obtain an emission in the white, by using the fluorescent property of phosphorus.
The general interest for this type of material is therefore very large.
Therefore, the invention in particular relates to making a thick GaN layer with good crystal quality (i.e., free of dislocations), which may optionally be used as a substrate, for example, for a subsequent epitaxy (by substrate, is meant in this text an entity that is able to bear the conditions of an epitaxy and that is flat enough to allow a good quality of the epitaxy, i.e., to grow an epitaxied layer that is flat, without cracks and with a low density of dislocations). Thick GaN films with low rates of dislocations, allow high-performance components to be made with longer life-times.
The high rate of dislocations is today one of the main technical limitations for improving performances of nitride-based components.
It is notably very difficult to obtain GaN substrates on the market of proper quality in a sufficient size for considering industrialization.
Making GaN layers by growing crystals on supporting substrates of the sapphire (Al2O3), SiC or silicon type is thus known.
However, lattice mismatches and differences in temperature behavior (different thermal expansion coefficients) between the different present materials are such that stresses are generated in the GaN layer, with appearance of defects of the dislocation type and other ones.
When the thickness of the epitaxied layer is too large for allowing it to expand or to retract, the cumulated elastic energy may even lead to cracking the GaN layer completely and thereby making it unusable.
The lattice mismatch problem may be limited by inserting between the supporting substrate and the GaN layer, intermediate layers (so-called “buffer layers”) with at least one layer typically consisting of at least two of the following elements: Al, Ga, As, In and N. These intermediate layers thus have the main purpose of reducing the impact of lattice parameter differences between the supporting substrate and the GaN layer.
By providing improvements to these so-called “hetero-epitaxy” methods on an SiC, a sapphire, or silicon substrate, it was thereby possible to make functional components.
However, certain electro-optical characteristics remain insufficient, such as emitted light power, leak current, life-time, etc.
Further, the making of such composite structures is long and expensive.
Moreover, with hetero-epitaxy, it is not always possible to totally get rid of the problems related to mechanical interaction (thermal expansion) between the substrate and the GaN layer.
European Patent EP 0 967 664 proposes another technique for making a thick GaN layer by homo-epitaxy on a GaAs supporting substrate, compatible with GaN epitaxy in terms of thermal expansion (the thermal expansion coefficient of GaAs being relatively close to that of GaN). The applied method comprises low temperature Epitaxial Lateral Overgrowth (also called ELOG) of a first GaN layer on a bulk GaAs substrate covered with an SiO2 mask in relief, followed by thickening of this first GaN layer by crystal growth.
After epitaxy, the GaAs substrate is suppressed by selective etching, for example, by using aqua regia, obtained by mixing nitric acid and hydrochloric acid.
Indeed, it may often be desirable to separate the GaN layer from the underlying structure in order to preserve the thick epitaxied GaN layer as a substrate layer, and this without having to sacrifice too much thickness from the GaN layer. For this purpose, a particularly selective etching of the substrate is a very efficient technique.
However, if selecting a GaAs substrate, it may be advisable to apply selective etching from a point of view of the mechanical behavior with temperature; it is less so from a crystallographic point of view. Indeed, the lattice mismatch with GaN is so large that it is necessary to make a sacrificial GaN layer on the GaAs substrate, prior to depositing a second GaN layer. For this purpose, the SiO2 mask in relief is first made so as to cause in the first GaN layer to be epitaxied laterally to the mask reliefs, confinement of a large number of defects, the latter thus playing the role of a sacrificial layer, before being used as a nucleation layer for the second GaN layer.
Now, making this last mask extends the manufacturing method in time, by adding additional steps thereto for forming a layer and for SiO2 photolithography according to predetermined patterns favorable to lateral growth of GaN.
These additional steps are further expensive to apply.
Finally, the first GaN layer, which is then epitaxied, remains of such poor crystal quality that it is necessary to epitaxy a strong layer of GaN in order to attain good crystal quality and a rate of dislocations less than 108 dislocations/cm2.
A goal of the invention is to make a thick GaN layer typically thicker than 10 micrometers, with good crystal quality (i.e., a number of dislocations less than 108 dislocations/cm2), by applying a faster and less expensive method.
Another goal is to avoid too large of losses of materials in the method for making the GaN layer.
For this purpose, the invention proposes a process for making a GaN substrate, comprising the following steps:                (a) transferring a first monocrystal GaN layer onto a supporting substrate;        (b) applying crystal growth for a second monocrystal GaN layer on the first layer;the first and second GaN layers thereby forming together said GaN substrate,said GaN substrate having a thickness of at least 10 micrometers, and        (c) removing at least one portion of the supporting substrate.        
Other characteristics of this process are the following:                the difference in thermal expansion coefficients between GaN and the material(s) making up the supporting substrate is between about 0.1·10−6 and about 2·10−6 K−1 for temperatures between about 20° C. and about 500° C.;        the supporting substrate has at least one surface layer in germanium or in an alloy made up of materials selected from the family of III-V materials; the supporting substrate may be in bulk GaAs;        prior to step (a), the process may comprise forming a protective coating on the supporting substrate; the protective coating may in particular be in a dielectric material, such as SiO2;        the protective coating is an encapsulation entirely surrounding the supporting substrate, or a protective layer formed on the face of the supporting substrate to be bonded to the first GaN layer; step (c) is a selective chemical etching of the encapsulation;        step (a) may comprise the formation of a bonding layer on one or both bonding surfaces, before putting the supporting substrate and the first GaN layer into contact; the bonding layer may in particular be in SiO2 or Si3N4;        step (c) comprises a selective chemical etching of the layer located at the interface between the supporting substrate and the first GaN layer, or selective chemical etching of at least one portion of the supporting substrate;        if the supporting substrate is in GaAs, the chemical etching agent is preferably aqua regia;        according to a preferred embodiment, the etching is achieved in the same enclosure as for the growth step (b) without having to handle the layers/supporting substrate assembly; the chemical etching agent is gaseous hydrochloric acid;        the process may further comprise, after step (c), a step of epitaxial growth on the GaN substrate;        the first GaN layer has a thickness between about 500 angstroms and about 1 micrometer;        the first GaN layer is initially comprised in an upper GaN layer of an initial structure, step (a) then allows bonding not only of the first layer but also of the whole initial structure to the supporting substrate, and the process further comprises, after step (a), a step for removing the initial structure located under the first GaN layer;        the step for removing the structure located under the first GaN layer is mainly applied by SMART CUT®, an implantation of atomic species having been applied beforehand into said upper layer to a thickness close to the thickness of said first GaN layer.        
The invention also proposes a GaN substrate on GaAs, characterized in that the GaN substrate has a density of dislocations less than about 108 dislocations/cm2 and has a thickness larger than about 10 micrometers.
The invention further proposes a GaN substrate having a density of dislocations less than about 108 dislocations/cm2 and a thickness comprised between 10 and 100 micrometers.